Photonics is the technology involved in communicating with light. It includes fiber optic technology (transmitting information as pulses of light though ultrapure glass fibers) as well as free-space optics (sending light signals through free-space, e.g., air). In telecommunication systems, fiber optics has now become the transmission medium of choice. A variety of optical switching systems is rapidly being developed to switch light signals between optical fibers. One type of optical switching arrangement is a fiber optic patch panel which is used m provide an arbitrary but fixed interconnection between two ordered arrays of optical fibers. In the prior art, there am three basic approaches to the implementation of a fiber optic patch panel.
First, one may use an optical fiber patch cord to interconnect the light signals from an input fiber array to an output fiber array. The fibers of such an interconnecting fiber patch cord being used to establish the desired interconnect pattern. This solution may be practical for the interconnection of small numbers of fibers. When many fibers, e.g., on the order of several hunted, have to be interconnected, thee arrangement and connectorization of the corresponding large interconnection bundle may become more impractical and difficult than other solutions.
Second, one may use waveguides, e.g., Lithium Niobate ( LiNbO.sub.3) couplers, to interconnect one-dimensional (1-D) fiber arrays. To implement a certain interconnect pattern, one has to lay out a corresponding routing pattern and transfer it to a wafer for the coupler waveguide fabrication. Again, this way of interconnecting fiber arrays is only practical if the number of fibers in the array is not too large. This is because the coupler waveguides cannot be densely spaced in the lateral direction, which results in increasing the longitudinal dimension to enable the couplers to perform the desired interconnect permutation. Since an interconnection chip is limited to the size of a wafer (several inches), the interconnection of several hundred fibers in one wafer does not seem feasible.
Third, one can send the light beams through free-space. Every input light beam is then deflected towards its desired output location. On their way, the light beams cross through each other in space without influencing each other. The free-space concept is naturally well suited for the interconnection of 2-D arrays and hence for the interconnection of a large number of fibers. Furthermore, the price of the main optical system does not increase as rapidly with the number of fibers as does the optical fiber patch cord solution.
A free-space, fiber optic patch panel (optical permutation interconnect arrangement) requires an input element to collimate and deflect the light signals from each input fiber and output element to deflect and focus the light signal for each output fiber.
One known free-space switching technique involves the use of small lenses (lenslets) in an off-axis configuration to perform both the collimate/focus and deflection functions. Unfortunately, the larger the deflection angle required, the more off-axis/off-center a lenslet has to be used. This requires lenslets of larger numerical apertures (a larger diameter for a given focal length).
For refractive lenslets, see FIG. 1, aberrations become more severe with higher apertures. Additionally, there is a waste of space because fabrication methods only allow one to put whole lenslets into an array instead of just the lens section(s) one really needs to use.
For diffractive lenslets (lithographically produced), see FIG. 2, the light efficiency goes down with higher numerical apertures due to limited resolution of the grating fabrication process. A higher numerical aperture means a smaller grating period at the edge of the lens. Given a minimum feature size, one may not be able to further subdivide the basic grating period in order to implement multilevel phase gratings needed for good light efficiency. Thus, there is a tradeoff between high aperture and high light efficiency. A further problem with the single off-axis diffractive lenslets is the strong wavelength dependence of both the focal length and the deflection angle.
For holographic lenslets (interferometrically produced diffractive lenslets), there is no problem with the light efficiency at large deflection angles (rather at small deflection angles), but the technology (recording materials and processing) is not yet (and may never be) mature enough to provide the required precision. Also, the holographic lenslets show the same strong wavelength dependence as the lithographically produced diffractive lenslets.
Optical interconnect arrangements have utilized singlet lenses which perform only the deflection task, excluding the collimation/focusing task. Prior art holographic singlet arrangements are described in H. Kobolla, F. Sauer and R. Volkel, "Holographic tandem arrays", Proc. SPIE, 1136 (1989) and B. Robertson, E. J. Restall, M. R. Taghizadeh, and A. C. Walker, "Space-variant holographic optical elements in dichromated gelatin," Appl. Opt., (1991) 2368. Diffractive singlet arrangements of the binary optics type performing both collimation/focusing and deflection tasks are described in J. Jahns and W.Daschner, "Optical cyclic shifter using diffractive lenslet arrays," Opt. Comm., 79 (1990) 407. The article entitled "Space-variant holographic optical elements for switching networks and general interconnects," by J. Schwider, W. Stork, N. Streibl, and R. Volkel, OSA Proceed. on Photonic Switching, Vol. 8, March 1991, pages 190-195, describes free-space optical permutation interconnect arrangements. One disclosed arrangement proposes a doublet made of two separate diffractive singlets to perform the collimation/focusing and deflection tasks. Undesirably, as previously noted, the pure holographic diffractive element solution exhibits strong wavelength dependence. Moreover, holographic technology does not now and may never provide the required precision needed for optical interconnect arrangements.
Thus, there is a continuing need to further improve the implementation of optical interconnect arrangements.